1. Field of the Invention
The present invention relates generally to enlarging the diameter of a subterranean borehole and, more specifically, to enlarging the borehole below a portion thereof which remains at a lesser diameter. The method and apparatus of the present invention effects such enlargement with enhanced stability of the bottomhole assembly, including a smoother and more controlled transition from the smaller pilot hole, which may or may not comprise the pass through diameter, to the enlarged bore diameter.
2. State of the Art
It is known to employ both eccentric and bi-center bits to enlarge a borehole below a tight or undersized portion thereof
An eccentric bit includes an extended or enlarged cutting portion which, when the bit is rotated about its axis, produces an enlarged borehole. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738.
A bi-center bit assembly employs two longitudinally-superimposed bit sections with laterally offset axes. The first axis is the center of the pass through diameter, that is, the diameter of the smallest borehole the bit will pass through. This axis may be referred to as the pass through axis. The second axis is the axis of the hole cut as the bit is rotated. This axis may be referred to as the drilling axis. There is usually a first, lower and smaller diameter pilot section employed to commence the drilling and rotation of the bit is centered about the drilling axis as the second, upper and larger diameter main bit section engages the formation to enlarge the borehole, the rotational axis of the bit assembly rapidly transitioning from the pass through axis to the drilling axis when the full diameter, enlarged borehole is drilled.
Rather than employing a one-piece drilling structure such as an eccentric bit or a bi-center bit to enlarge a borehole below a constricted or reduced-diameter segment, it is also known to employ an extended bottomhole assembly (extended bi-center assembly) with a pilot bit at the distal end thereof and a reamer assembly some distance above. This arrangement permits the use of any standard bit type, be it a rock bit or a drag bit, as the pilot bit, and the extended nature of the assembly permits greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will take the path intended for the borehole. This aspect of an extended bottomhole assembly is particularly significant in directional drilling.
While all of the foregoing alternative approaches can be employed to enlarge a borehole below a reduced-diameter segment, the pilot bit with reamer assembly has proven to be the most effective overall. The assignee of the present invention has, to this end, designed as reaming structures so-called "reamer wings" in the very recent past, which reamer wings generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof, and a tong die surface at the bottom thereof, also with a threaded connection. The upper mid-portion of the reamer wing includes one or more longitudinally-extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive (also termed "superhard") cutting elements, commonly termed "PDC's" (for Polycrystalline Diamond Compacts). The lower mid-portion of the reamer wing may include a stabilizing pad having an arcuate exterior surface of the same or slightly smaller radius than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizer pad is characteristically placed on the opposite side of the body with respect to the reamer wing blades so that the reamer wing will ride on the pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut.
While the aforementioned reamer wing design enjoyed some initial success, it was recognized that the device as constructed might not effectively and efficiently address the problem or task of achieving a rapid transition from pass through to full hole or "drill" diameter which closely tracks the path of the pilot bit and which does not unduly load the blades or bottomhole assembly during the transition. Since a reamer wing may have to reestablish a full diameter borehole multiple times during its drilling life in a single borehole, due to washouts and doglegs of the pilot hole, a rapid transitioning ability when reaming is re-started as well as a robust design which can accommodate multiple transitions without significant damage was recognized as a desirable characteristic and design modification. U.S. Pat. No. 5,497,842, assigned to the assignee of the present invention and incorporated herein for all purposes by this reference, discloses the use of so-called "secondary" blades on the reamer wing to speed the transition from pass through to drill diameter with reduced vibration and borehole eccentricity.
While the improvement of the '842 patent has proven significant, it has been recognized by the inventors herein that further improvements in the overall stability of the bottomhole assembly, including transitioning from pass through to drill diameter, would be highly desirable. One problem the prior art reamer assembly designs have experienced is undue vibration and even so-called bit "whirl," despite the focused or directed force vector acting on the reaming assembly and the presence of the stabilization pad. These undesirable phenomena appear to be related to the configuration of the stabilization pad (illustrated in FIG. 5 of the '842 patent), which engages the borehole wall axially and circumferentially under the radially-directed resultant force vector of the reamer wing as the assembly drills ahead in the pilot hole, due to the pad's abrupt radial projection from the reamer wing body. Furthermore, it has been observed that the entire bottomhole reaming assembly as employed in the prior art for straight-hole drilling with a rotary table or top drive often experiences pipe "whip" due to lack of sufficient lateral or radial stabilization above the reamer wing. In addition, reaming assemblies driven by downhole steerable motors for so-called directional or navigational drilling have experienced problems with stability under the lateral forces generated by the reamer wing so as to make it difficult to maintain the planned borehole trajectory.
In order to provide the reader with a better understanding of the problems associated with prior art reaming assemblies and to better appreciate the advantages of the present invention, FIGS. 1 through 3 herein depict an exemplary prior art bi-center bottomhole assembly 10 in which the reamer wing disclosed in U.S. Pat. No. 5,497,842 is employed.
Commencing with FIG. 1 and moving from the top to the bottom of the assembly 10, one or more drill collars 12 are suspended from the distal end of a drill string extending to the rig floor at the surface. Pass through stabilizer 14 (optional) is secured to drill collar 12, stabilizer 14 being sized equal to or slightly smaller than the pass through diameter of the bottomhole assembly 10, which may be defined as the smallest diameter borehole through which the assembly may move longitudinally. Another drill collar 16 (or other drill string element such as an MWD tool housing or pony collar) is secured to the bottom of stabilizer 14, below which reamer wing 100 including a stabilization pad 118 is secured via tool joint 18. Another API joint 22 is located at the bottom of the reamer wing 100. An upper pilot stabilizer 24, secured to reamer wing 100, is of an outer diameter (O.D.) equal to or slightly smaller than that of the pilot bit at the bottom of the assembly 10. Yet another, smaller diameter drill collar 26 is secured to the lower end of pilot stabilizer 24, followed by a lower pilot stabilizer 28 to which is secured pilot bit 30. Pilot bit 30 may be either a rotary drag bit or a tri-cone, so-called "rock bit". The bottomhole assembly as described is exemplary only, it being appreciated by those of ordinary skill in the art that many other assemblies and variations may be employed.
It should be noted that there is an upper lateral displacement 32 between the axis of pass through stabilizer 14 and that of reamer wing 100, which displacement is provided by the presence of drill collar 16 therebetween and which promotes passage of the assembly 10, and particularly the reamer wing 100, through a borehole segment of the design pass through diameter.
For purposes of discussion, the following exemplary dimensions may be helpful in understanding the relative sizing of the components of the assembly for a particular pass through diameter, pilot diameter and drill diameter. For a pass through diameter of 10.625 inches, a pilot diameter of 8.500 inches and a maximum drill diameter of 12.250 inches (the full bore diameter drilled by reamer wing 100) would normally be specified. In the bottomhole assembly 10, for the above parameters:
(a) drill collar 12 may be an eight inch drill collar; PA1 (b) drill collar 16 may be a thirty foot, eight inch drill collar; PA1 (c) drill collar 26 may be a fifteen foot, 63/4 inch drill collar; and PA1 (d) pilot bit 30 is an 81/2 inch bit.
In pass through condition, shown in FIG. 1, the assembly 10 is always in either tension or compression, depending upon the direction of travel, as shown by arrow 34. Contact of the assembly with the borehole wall 50 is primarily through pass through stabilizer 14 and reamer wing 100. The assembly 10 is not normally rotated while in pass through condition.
FIG. 2 depicts start-up condition of assembly 10, wherein assembly 10 is rotated by application of torque as shown by arrow 36 as weight-on-bit (WOB) is also applied to the string, as shown by arrow 38. As shown, pilot bit 30 has drilled ahead into the uncut formation to a depth approximating the position of upper pilot stabilizer 24, but reamer wing 100 has yet to commence enlarging the borehole to drill diameter. As shown at 32 and at 40, the axis of reamer wing 100 is laterally displaced from those of both pass through stabilizer 14 and upper pilot stabilizer 24. In this condition, the reamer wing 100 has not yet begun its transition from being centered about a pass through center line to its drilling mode center line which is aligned with that of pilot bit 30.
FIG. 3 depicts the normal drilling mode of bottomhole assembly 10, wherein torque 36 and WOB 38 are applied. Upper displacement 32 may remain as shown, but generally is eliminated under all but the most severe drilling conditions. Lower displacement 40 has been eliminated as reamer wing 100 is rotating about the same axis as pilot bit 30 in cutting the borehole to full drill diameter. It is readily apparent from FIG. 3 that concentric stabilizer 14 (if employed) performs only a nominal stabilization function once enlargement of the borehole is fully underway and stabilizer 14 has passed into the enlarged segment of the borehole. In such circumstances, the aforementioned drill string "whip" is experienced due to effective contact of the string with the borehole wall being limited to only one lateral or radial location.
It is also known to employ expandable concentric stabilizers to effect better stabilization of the bottomhole assembly in the enlarged borehole, the diameter of which stabilizers may be increased by string manipulation or hydraulically once the stabilizer has reached an enlarged portion of the borehole, one such device being disclosed in U.S. Pat. No. 4,854,403, assigned to the assignee of the present invention. Such devices, however, are relatively complex and expensive, and may fail to contract after expansion, impeding or preventing the trip out of the borehole.
It is also readily apparent from FIG. 3 that prior art stabilization pad 118 of the configuration as previously described is forced into the wall of the pilot hole, thus engaging it both axially and circumferentially as the assembly rotates and follows the pilot bit, promoting unwanted vibration and possibly inducing whirl of the assembly.